This invention is directed to a thermal-mechanical process, and to the product thereof, for treating carbon or low alloy steels to improve their strength and toughness. Metallurgists have for year attempted to improve the mechanical properties of rolled steel products through combinations of thermal-mechanical treatments and through specially engineered quenching cycles. A major failure of these practices has been the absence of a commercially viable practice. Continuum rolling, as represented by U.S. Pat. Nos. 3,645,801 and 3,826,691, is a good example of a recent thermal-mechanical practice to improve mechanical properties of rolled steel products, but which has limited commercial application.
Continuum rolling, as defined by such patents, covers a thermal-mechanical process wherein a steel work-piece is subjected to a rolling sequence which includes rolling said workpiece while it is austenitic, followed by further rolling within the austenite-ferrite, and ferrite regions to product a steel which is highly textured in microstructure.
Controlled rolling is a variation of continuum rolling in that all working is concluded in the austenite-ferrite region. U.S. Pat. No. 3,806,378 graphically illustrates the controlled rolling treatment sequence along with other thermal-mechanical treatments which have been developed over the years. A major drawback of these thermal-mechanical treatments is the inherent time delays necessary to equalize temperatures of the steel workpiece. Though the properties of the resulting steel reveal useful and valuable characteristics, the practices are not suited for commercial mills where tonnage production is a necessary factor to develop a marketable product.
There have been other patented developments in the area of thermal-mechanical treatments. For example, U.S. Pat. No. 4,088,511 teaches a thermal-mechanical treatment for improving the properties of carbon and low alloy steels. The method taught therein includes rapidly heating to partially austenitize a steel workpiece to develop a ferrite-austenite mixture, quenching to render the austenite metastable, equalizing the temperature throughout the steel workpiece, followed by mechanical working. Another example of a thermal-mechanical treatment is taught in U.S. Pat. No. 4,040,872. The patent discloses a process which includes austenitizing, quenching, mechanical working below the lower critical temperature, and stress relieving.
Efforts by metallurgists engaged in research have not been restricted to purely thermal-mechanical treatments; they have sought to improve properties and structure through post-working heat treatments.
Several patents, described below, teach heat treating methods for developing differences in through thickness properties and structures. U.S. Pat. No. 4,165,246 teaches a method for austenitizing and quenching thick-walled steel pipe to develop martensite in the outer layers thereof but not in the non-quenched wall parts, followed by tempering the martensitic outer layers by residual heat from the inner wall layers. U.S. Pat. Nos. 4,016,009 and 4,016,015 teach similar practices in which a steel rod or bar are hot rolled, surface quenched after the hot mill finishing stand to produce a surface layer of martensite or bainite, followed by slow cooling to develop a modified core structure and a tempered surface layer.
The present invention is the culmination of a research investigation to improve the properties of carbon or low alloy steels by means commercially attractive to steel producers. Such investigation was successful as will be made apparent by the description which follows.